The present invention relates to a method of producing olefin polymers or copolymers of high stereospecificity at a high rate of yields with a broad molecular weight distribution.
In general, polypropylene has characteristics of enhanced strength and heat-resistance at a broader molecular weight distribution and higher stereospecificity. As such, many attempts were made to broaden the molecular weight distribution and improve stereospecificity. Recently, Mitsui Petrochemical Industries, Ltd. and other well-known European companies have developed a polymerization method, by means of using specific silicon compounds, for polymers of high yields and stereospecificity (e.g., Korean Pat. Pub. No. 92-2488, U.S. Pat. No. 4,990,479, EP Laid-Open No. 350,170A, Chinese Pat. No. 1,040,379), as compared to those of the conventional methods. However, it was very difficult to produce polymers of the broad molecular weight and high MFR (Melt Flow Rate) due to the low hydrogen response. Further, in order to broaden the molecular weight distribution of the polymers, it was common to use a method of producing and then mixing olefin polymers of different molecular weight distributions from one another in a number of polymerization reactors. Yet, this method takes too much time and has disadvantages in that the products thereof are non-homogeneous.
In a recent report, from Mitsui Petrochemical of Japan(publication No. 93-665, of Korea Patent), a method has been proposed in which an olefin polymer with a wider distribution of molecular weights is produced by the use of two particular electron donors, from which homopolyolefins having the ratio of the melt flow rate (MFR) if greater than 31.6 are respectively polymerized in the same polymerization conditions. In this case, however, the catalyst's activity is reduced too low and the stereospecificity of polymer is too low for it to be commercially useful.
Meanwhile, many other techniques are known to produce polymers or copolymers of high stereospecificity by the use of solid complex titanium catalysts containing magnesium with electron donors, and also titanium and a halogen, as catalyst for polymerization or copolymerization of .alpha.-olefin which contains more than three atoms of carbon (e.g. Japanese, Pts. Laid-Open Nos. S48-16986 and S48-16987, Ger. Pts. Laid-Open Nos. 2,153,520; 2,230,672; 2,230,728; 2,230,752; and 2,553,104)
These references reveal the use of mixture components of particular catalysts and the process for forming these catalysts. As is well known, the characteristics of these catalysts, containing solid complex titanium, vary from catalyst to catalyst, in accordance with the different mixtures of components, different combinations of processes for formation, and the different combinations of these conditions. Therefore, it is very difficult to predict what effects can be obtained from a catalyst produced under a given set of conditions. Often, a catalyst having undesirable effect is produced. It is also often true that such characteristics as the activity of the catalyst or the stereospecificity of the polymer do not turn out to be adequate even if the catalyst is made under proper conditions, if proper external electron donors are not used.
The solid complex titanium catalyst containing magnesium, titanium, and halogen is no exception. In polymerizing or copolymerizing .alpha.-olefin containing more than three atoms of carbon, in the presence of hydrogen and with the use of a catalyst composed of titanium and an organometallic compound of metals belonging to Groups I through IV on the periodic table of elements, if a co-catalyst composed of titanium trichloride obtained by reducing titanium tetrachloride using metallic aluminum, hydrogen, or an organic aluminum compound is used, along with such electron donors as are known to restrain the formation of amorphous copolymer, the effects vary depending upon the electron donors used. The cause is accepted to be that the electron donors are not mere inert additives, rather, they combine with the magnesium and titanium compounds electronically and sterically, thereby fundamentally altering the structure of the solid complex catalyst.